Compressed gas-insulated transmission systems are well known and consist of a central tubular conductor concentric with an outer tubular sheath or housing, with compressed gas, usually an electronegative gas such as SF.sub.6, filling the housing interior. Spacer insulators at intervals maintain the concentricity of the concentric tubes. Such systems have inherent advantages for transmission of electrical power due to high current capacity, low losses, and low capacitance of the cable. Such systems and cables are described in U.S. Pat. Nos. 4,132,855, entitled SUPPORT INSULATOR FOR GAS-FILLED HIGH-VOLTAGE TRANSMISSION LINE, issued Jan. 2, 1979, and 3,982,806, entitled PLUG-IN ELECTRIC CONTACT WITH IMPROVED CONTACT FINGER SUPPORT AND SHIELDING, issued Sept. 28, 1976. These cables can be used for underwater application as well for applications in which they are above ground or buried in the ground. Thus the cable can be advantageously used for lake crossings and for connection of power to and from off-short generators or installations, as shown in U.S. Pat. No. 3,794,849 entitled POWER TRANSMISSION SYSTEM FOR CONNECTING FLOATING POWER PLANT TO STATIONARY CONDUCTORS, issued Feb. 26, 1974.
A problem when submerging cable in water is that water may leak into the cable. A small leak of the insulating gas into the water would be harmless to the cable and leak rates of 1% or 2% per year are acceptable. Gas is simply added back into the system as part of the maintenance procedure. However, a small leak of water into the cable would probably cause a failure if the water collected as liquid rather than being evaporated into the gas space. Therefore, in underwater applications, it is desirable to maintain the gas pressure within the housing at a value higher than that of the water. Compressed gas-insulated systems using SF.sub.6 commonly operate at about 60 p.s.i.g. The wall thickness of the outer housing, usually of aluminum, is designed to safely contain this pressure. Since this is the pressure of water at about a 120 foot depth, it follows that a safe, leak-proof depth for such a system would be about 100 feet.
To use the cable in depths of 200 feet, however, would require an increase in internal pressure to about 120 p.s.i.g. to ensure that the internal gas pressure would prevent leaking of water into the housing. That would require a heavier wall thickness for the outer housing if the outer housing is to safely contain this higher pressure. The increased wall thickness increases the weight and cost of the housing, even though the increased thickness is not needed in the parts of the cable that are exposed to lower water pressure at cable depths less than 100 feet or atmosphere only.